Engine emission control systems may include one or more exhaust catalysts to address the various exhaust components. These may include, for example, three-way catalysts, NOx storage catalysts, light-off catalysts, SCR catalysts, etc. Engine exhaust catalysts may utilize periodic regeneration to restore catalytic activity and reduce catalyst oxidation. For example, catalysts may be regenerated by injecting sufficient fuel to produce a rich environment and reduce the amount of oxygen stored at the catalyst. Because fuel consumed during catalyst regeneration can degrade engine fuel economy, various catalyst regeneration strategies have been developed.
One example approach is shown by Georigk et al. in U.S. Pat. No. 6,969,492. Therein, an emission control device includes catalytic converter stages generated by at least two catalysts arranged in series. Specifically, the catalytic stages include a three-way catalyst arranged in series with (e.g., upstream of) a NOx reduction catalyst. The different ammonia storage performance of the different catalysts enables NOx reduction to be improved and reduces the need for catalyst regeneration. Another example approach is shown by Eckhoff et al. in WO 2009/080152. Therein, an engine exhaust system includes multiple NOx storage catalysts with an intermediate SCR catalyst, and an exhaust air-to-fuel ratio is continually alternated between rich and lean phases based on differences between an air-to-fuel ratio upstream of a first NOx storage catalyst and an air-to-fuel ratio downstream of a second NOx storage catalyst.
However, the inventors herein have identified potential issues with such approaches. For example, the inventors have recognized that the regeneration control may degrade during operations when one or more cylinders may be deactivated by shutting off fuel to the cylinders during a vehicle drive cycle. During these operations, while the engine is deactivated and fuel is shut-off to improve drivability and performance, the engine may continue to spin. This spinning pumps air over an exhaust three-way catalyst, causing the catalyst to become oxidized and degrading its ability to reduce NOx when the engine is reactivated. And while enrichment can be used to quickly regenerate the three-way catalyst upon engine reactivation, the enrichment leads to a fuel penalty. Another consequence of engine pumping air over the catalyst may include an increase in catalyst temperature, which further degrades catalyst performance.
In one example, a method may include selectively deactivating one or more engine cylinders via deactivatable fuel injectors during a selected condition; and during the cylinder deactivation, injecting water at the one or more deactivated engine cylinders to reduce oxygenation of a first exhaust catalyst.
Events during which one or more cylinders may be deactivated may include transmission shifting during automatic and manual operations, deceleration fuel shut-off (DFSO), misfire failure mode effects management (misfire FMEM), and engine speed flare control during start-stop transients, for example. In this way, by injecting water and reducing catalyst oxidation during a cylinder deactivation event, a fuel penalty from enrichment during cylinder reactivation may be reduced while maintaining a required NOx emission level. Additionally, water injection during a cylinder deactivation event may reduce excessive increase in catalyst temperature. By reducing catalyst temperature, optimal catalyst performance may be achieved. Further, injecting water at the deactivated cylinders facilitates reduction in the amount of hydrocarbons in the exhaust through a steam reforming process across the first exhaust catalyst, upon fuel reactivation. Therefore, water injection, in addition to reducing oxidation and temperature of the exhaust catalyst, can decrease hydrocarbon emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.